Substrate holder and method of manufacturing a substrate holder

ABSTRACT

An object holder ( 100 ) for a lithographic apparatus has a main body ( 400 ) having a surface ( 400   a ). A plurality of burls ( 406 ) to support an object are formed on the surface or in apertures of a thin-film stack ( 410, 440, 450 ). At least one of the burls is formed by laser-sintering. At least one of the burls formed by laser-sintering may be a repair of a damaged burl previously formed by laser-sintering or another method.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application61/594,857, which was filed on Feb. 3, 2012, U.S. provisionalapplication 61/621,648, which was filed on Apr. 9, 2012 and U.S.provisional application 61/621,660, which was filed on Apr. 9, 2012 andwhich are incorporated herein in its entirety by reference.

FIELD

The present invention relates to a substrate holder, a lithographicapparatus, a device manufacturing method, and a method of manufacturinga substrate holder.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

It has been proposed to immerse the substrate in the lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the final element ofthe projection system and the substrate. In an embodiment, the liquid isdistilled water, although another liquid can be used. An embodiment ofthe present invention will be described with reference to liquid.However, another fluid may be suitable, particularly a wetting fluid, anincompressible fluid and/or a fluid with higher refractive index thanair, desirably a higher refractive index than water. Fluids excludinggases are particularly desirable. The point of this is to enable imagingof smaller features since the exposure radiation will have a shorterwavelength in the liquid. (The effect of the liquid may also be regardedas increasing the effective numerical aperture (NA) of the system andalso increasing the depth of focus.) Other immersion liquids have beenproposed, including water with solid particles (e.g. quartz) suspendedtherein, or a liquid with a nano-particle suspension (e.g. particleswith a maximum dimension of up to 10 nm). The suspended particles may ormay not have a similar or the same refractive index as the liquid inwhich they are suspended. Other liquids which may be suitable include ahydrocarbon, such as an aromatic, a fluorohydrocarbon, and/or an aqueoussolution.

SUMMARY

In a conventional lithography apparatus, the substrate to be exposed maybe supported by a substrate holder which in turn is supported by asubstrate table. The substrate holder is often a flat rigid disccorresponding in size and shape to the substrate (although it may have adifferent size or shape). It has an array of projections, referred to asburls or pimples, projecting from at least one side. In an embodiment,the substrate holder has an array of projections on two opposite sides.In this case, when the substrate holder is placed on the substratetable, the main body of the substrate holder is held a small distanceabove the substrate table while the ends of the burls on one side of thesubstrate holder lie on the surface of the substrate table. Similarly,when the substrate rests on the top of the burls on the opposite side ofthe substrate holder, the substrate is spaced apart from the main bodyof the substrate holder. One purpose of this is to help prevent aparticle (i.e. a contaminating particle such as a dust particle) whichmight be present on either the substrate table or substrate holder fromdistorting the substrate holder or the substrate. Since the totalsurface area of the burls is only a small fraction of the total area ofthe substrate or substrate holder, it is highly probable that anyparticle will lie between burls and its presence will have no effect.

Due to the high accelerations experienced by the substrate in use of ahigh-throughput lithographic apparatus, it is not sufficient to allowthe substrate simply to rest on the burls of the substrate holder. It isclamped in place. Two methods of clamping the substrate in place areknown—vacuum clamping and electrostatic clamping. In vacuum clamping,the space between the substrate holder and substrate and optionallybetween the substrate table and substrate holder are partially evacuatedso that the substrate is held in place by the higher pressure of gas orliquid above it. Vacuum clamping however may not be feasible where thebeam path and/or the environment near the substrate or substrate holderis kept at a low or very low pressure, e.g. for extreme ultraviolet(EUV) radiation lithography. In this case, it may not be possible todevelop a sufficiently large pressure difference across the substrate(or substrate holder) to clamp it. Electrostatic clamping can thereforebe used in such a circumstance (or in other circumstances). Inelectrostatic clamping, an electrode provided on the substrate tableand/or substrate holder is raised to a high potential, e.g. 10 to 5000V, and electrostatic forces attract the substrate. Thus another purposeof the burls is to space the substrate, substrate holder and substratetable apart in order to enable electrostatic clamping.

Burls can be used in various other places within a lithographicapparatus, for example in a support for a patterning device such as amask, in a gripper of substrate or patterning device handling apparatus,and/or a reticle clamp. Burls in different places may have differentrequirements as to one or more of their dimensions and other physicalproperties. A different method of manufacture may apply. In many cases,damage to one or more burls may necessitate replacement of an entirecomponent.

It is desirable, for example, to provide an object holder for use in alithographic apparatus, the holder having burls, e.g. of differentshapes, sizes and/or compositions, and a method of manufacturing anobject holder having such burls.

According to an aspect of the invention, there is provided a method ofmanufacturing an object holder for use in a lithographic apparatus, themethod comprising: providing a main body having a surface; and forming aplurality of burls on the surface, the burls projecting from the surfaceand having end surfaces to support an object, wherein forming at leastpart of at least one of the burls comprises laser-sintering.

According to an aspect of the invention, there is provided an objectholder for use in a lithographic apparatus, the object holdercomprising: a main body having a surface; and a plurality of burlsprovided on the surface and having end surfaces to support an object,wherein at least part of at least one of the burls has been formed bylaser-sintering.

According to an aspect of the invention, there is provided alithographic apparatus, comprising: a support structure configured tosupport a patterning device; a projection system arranged to project abeam patterned by the patterning device onto a substrate; and asubstrate holder arranged to hold the substrate, the substrate holderbeing as described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIGS. 2 and 3 depict a liquid supply system for use in a lithographicprojection apparatus;

FIG. 4 depicts a further liquid supply system for use in a lithographicprojection apparatus;

FIG. 5 depicts, in cross-section, a barrier member which may be used inan embodiment of the present invention as an immersion liquid supplysystem;

FIG. 6 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 7 is a more detailed view of the apparatus 4100;

FIG. 8 is a more detailed view of the source collector apparatus SO ofthe apparatus of FIGS. 6 and 7;

FIG. 9 depicts in cross-section a substrate table and a substrate holderaccording to an embodiment of the invention;

FIGS. 10A to 10E depict steps in a method of manufacturing a substrateholder according to an embodiment of the invention;

FIG. 11 depicts a substrate holder according to an embodiment of theinvention;

FIG. 12 depicts a support structure for a patterning device according toan embodiment of the invention;

FIG. 13 depicts in plan a part of a support structure according to anembodiment of the invention;

FIG. 14 depicts the support structure of FIG. 13 in cross-section;

FIG. 15 depicts a substrate handler and gripper according to anembodiment of an invention;

FIG. 16 depicts a substrate table according to an embodiment of theinvention;

FIGS. 17A to C depict steps in a method of repairing a substrate holderaccording to an embodiment of the invention; and

FIG. 18A to C depict steps in a method of repairing a substrate holderaccording to an embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus comprises:

-   -   an illumination system (illuminator) IL configured to condition        a radiation beam B (e.g. UV radiation, DUV radiation or EUV        radiation);    -   a support structure (e.g. a mask table) MT constructed to        support a patterning device (e.g. a mask) MA and connected to a        first positioner PM configured to accurately position the        patterning device in accordance with certain parameters;    -   a substrate table (e.g. a wafer table) WT constructed to hold a        substrate (e.g. a resist-coated wafer) W and connected to a        second positioner PW configured to accurately position the        substrate in accordance with certain parameters; and    -   a projection system (e.g. a refractive projection lens system)        PS configured to project a pattern imparted to the radiation        beam B by patterning device MA onto a target portion C (e.g.        comprising one or more dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure MT holds the patterning device. The supportstructure MT holds the patterning device in a manner that depends on theorientation of the patterning device, the design of the lithographicapparatus, and other conditions, such as for example whether or not thepatterning device is held in a vacuum environment. The support structureMT can use mechanical, vacuum, electrostatic or other clampingtechniques to hold the patterning device. The support structure MT maybe a frame or a table, for example, which may be fixed or movable asrequired. The support structure MT may ensure that the patterning deviceis at a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device”.

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The terms “projection system” used herein should be broadly interpretedas encompassing any type of system, including refractive, reflective,catadioptric, magnetic, electromagnetic and electrostatic opticalsystems, or any combination thereof, as appropriate for the exposureradiation being used, or for other factors such as the use of animmersion liquid or the use of a vacuum. Any use of the term “projectionlens” herein may be considered as synonymous with the more general term“projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two or more substratesupport structures, such as substrate stages or substrate tables, and/ortwo or more support structures for patterning devices. In an apparatuswith multiple substrate stages, all the substrate stages can beequivalent and interchangeable. In an embodiment, at least one of themultiple substrate stages is particularly adapted for exposure steps andat least one of the multiple substrate stages is particularly adaptedfor measurement or preparatory steps. In an embodiment of the inventionone or more of the multiple substrate stages is replaced by ameasurement stage. A measurement stage includes at least part one ormore sensor systems such as a sensor detector and/or target of thesensor system but does not support a substrate. The measurement stage ispositionable in the projection beam in place of a substrate stage or asupport structure for a patterning device. In such apparatus theadditional stages may be used in parallel, or preparatory steps may becarried out on one or more stages while one or more other stages arebeing used for exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AM configured to adjust theangular intensity distribution of the radiation beam. Generally, atleast the outer and/or inner radial extent (commonly referred to asσ-outer and σ-inner, respectively) of the intensity distribution in apupil plane of the illuminator can be adjusted. In addition, theilluminator IL may comprise various other components, such as anintegrator IN and a condenser CO. The illuminator may be used tocondition the radiation beam, to have a desired uniformity and intensitydistribution in its cross-section. Similar to the source SO, theilluminator IL may or may not be considered to form part of thelithographic apparatus. For example, the illuminator IL may be anintegral part of the lithographic apparatus or may be a separate entityfrom the lithographic apparatus. In the latter case, the lithographicapparatus may be configured to allow the illuminator IL to be mountedthereon. Optionally, the illuminator IL is detachable and may beseparately provided (for example, by the lithographic apparatusmanufacturer or another supplier).

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., mask table) MT, and ispatterned by the patterning device. Having traversed the patterningdevice MA, the radiation beam B passes through the projection system PS,which focuses the beam onto a target portion C of the substrate W.Substrate W is held on the substrate table WT by a substrate holderaccording to an embodiment of the present invention and describedfurther below. With the aid of the second positioner PW and positionsensor IF (e.g. an interferometric device, linear encoder or capacitivesensor), the substrate table WT can be moved accurately, e.g. so as toposition different target portions C in the path of the radiation beamB. Similarly, the first positioner PM and another position sensor (whichis not explicitly depicted in FIG. 1) can be used to accurately positionthe patterning device MA with respect to the path of the radiation beamB, e.g. after mechanical retrieval from a mask library, or during ascan. In general, movement of the support structure MT may be realizedwith the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which form part of the firstpositioner PM. Similarly, movement of the substrate table WT may berealized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the support structure MT may be connected to ashort-stroke actuator only, or may be fixed. Patterning device MA andsubstrate W may be aligned using patterning device alignment marks M1,M2 and substrate alignment marks P1, P2. Although the substratealignment marks as illustrated occupy dedicated target portions, theymay be located in spaces between target portions (these are known asscribe-lane alignment marks). Similarly, in situations in which morethan one die is provided on the patterning device MA, the patterningdevice alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the support structure MT and the substrate table WT arekept essentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the support structure MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the supportstructure MT may be determined by the (de-) magnification and imagereversal characteristics of the projection system PS. In scan mode, themaximum size of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the support structure MT is kept essentiallystationary holding a programmable patterning device, and the substratetable WT is moved or scanned while a pattern imparted to the radiationbeam is projected onto a target portion C. In this mode, generally apulsed radiation source is employed and the programmable patterningdevice is updated as required after each movement of the substrate tableWT or in between successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

In many lithographic apparatuses, a fluid, in particular a liquid, isprovided between the final element of the projection system using aliquid supply system IH to enable imaging of smaller features and/orincrease the effective NA of the apparatus. An embodiment of theinvention is described further below with reference to such an immersionapparatus, but may equally be embodied in a non-immersion apparatus.Arrangements to provide liquid between a final element of the projectionsystem and the substrate can be classed into at least two generalcategories. These are the bath type arrangement and the so calledlocalized immersion system. In the bath type arrangement substantiallythe whole of the substrate and optionally part of the substrate table issubmersed in a bath of liquid. The localized immersion system uses aliquid supply system in which liquid is only provided to a localizedarea of the substrate. In the latter category, the space filled byliquid is smaller in plan than the top surface of the substrate and thearea filled with liquid remains substantially stationary relative to theprojection system while the substrate moves underneath that area.Another arrangement, to which an embodiment of the invention isdirected, is the all wet solution in which the liquid is unconfined. Inthis arrangement substantially the whole top surface of the substrateand all or part of the substrate table is covered in immersion liquid.The depth of the liquid covering at least the substrate is small. Theliquid may be a film, such as a thin-film, of liquid on the substrate.

Four different types of localized liquid supply systems are illustratedin FIGS. 2 to 5. Any of the liquid supply devices of FIGS. 2 to 5 may beused in an unconfined system; however, sealing features are not present,are not activated, are not as efficient as normal or are otherwiseineffective to seal liquid to only the localized area.

One of the arrangements proposed for a localized immersion system is fora liquid supply system to provide liquid on only a localized area of thesubstrate and in between the final element of the projection system andthe substrate using a liquid confinement system (the substrate generallyhas a larger surface area than the final element of the projectionsystem). One way which has been proposed to arrange for this isdisclosed in PCT patent application publication no. WO 99/49504. Asillustrated in FIGS. 2 and 3, liquid is supplied by at least one inletonto the substrate, desirably along the direction of movement of thesubstrate relative to the final element, and is removed by at least oneoutlet after having passed under the projection system. That is, as thesubstrate is scanned beneath the element in a −X direction, liquid issupplied at the +X side of the element and taken up at the −X side.

FIG. 2 shows the arrangement schematically in which liquid is suppliedvia inlet and is taken up on the other side of the element by outletwhich is connected to a low pressure source. The arrows above thesubstrate W illustrate the direction of liquid flow, and the arrow belowthe substrate W illustrates the direction of movement of the substratetable. In the illustration of FIG. 2 the liquid is supplied along thedirection of movement of the substrate relative to the final element,though this does not need to be the case. Various orientations andnumbers of in-and out-lets positioned around the final element arepossible, one example is illustrated in FIG. 3 in which four sets of aninlet with an outlet on either side are provided in a regular patternaround the final element. Arrows in liquid supply and liquid recoverydevices indicate the direction of liquid flow.

A further immersion lithography solution with a localized liquid supplysystem is shown in FIG. 4. Liquid is supplied by two groove inlets oneither side of the projection system PS and is removed by a plurality ofdiscrete outlets arranged radially outwardly of the inlets. The inletsand outlets can be arranged in a plate with a hole in its center andthrough which the projection beam is projected. Liquid is supplied byone groove inlet on one side of the projection system PS and removed bya plurality of discrete outlets on the other side of the projectionsystem PS, causing a flow of a thin-film of liquid between theprojection system PS and the substrate W. The choice of whichcombination of inlet and outlets to use can depend on the direction ofmovement of the substrate W (the other combination of inlet and outletsbeing inactive). In the cross-sectional view of FIG. 4, arrowsillustrate the direction of liquid flow in to inlets and out of outlets.

Another arrangement which has been proposed is to provide the liquidsupply system with a liquid confinement member which extends along atleast a part of a boundary of the space between the final element of theprojection system and the substrate table. Such an arrangement isillustrated in FIG. 5. The liquid confinement member is substantiallystationary relative to the projection system in the XY plane, thoughthere may be some relative movement in the Z direction (in the directionof the optical axis). A seal is formed between the liquid confinementmember and the surface of the substrate. In an embodiment, a seal isformed between the liquid confinement member and the surface of thesubstrate and may be a contactless seal such as a gas seal. Such asystem is disclosed in United States patent application publication no.US 2004-0207824.

The fluid handling structure 12 includes a liquid confinement member andat least partly contains liquid in the space 11 between a final elementof the projection system PS and the substrate W. A contactless seal 16to the substrate W may be formed around the image field of theprojection system so that liquid is confined within the space betweenthe substrate W surface and the final element of the projection systemPS. The space is at least partly formed by the fluid handling structure12 positioned below and surrounding the final element of the projectionsystem PS. Liquid is brought into the space below the projection systemand within the fluid handling structure 12 by liquid inlet 13. Theliquid may be removed by liquid outlet 13. The fluid handling structure12 may extend a little above the final element of the projection system.The liquid level rises above the final element so that a buffer ofliquid is provided. In an embodiment, the fluid handling structure 12has an inner periphery that at the upper end closely conforms to theshape of the projection system or the final element thereof and may,e.g., be round. At the bottom, the inner periphery closely conforms tothe shape of the image field, e.g., rectangular, though this need not bethe case.

In an embodiment, the liquid is contained in the space 11 by a gas seal16 which, during use, is formed between the bottom of the fluid handlingstructure 12 and the surface of the substrate W. The gas seal is formedby gas, e.g. air, synthetic air, N₂ or another inert gas. The gas in thegas seal is provided under pressure via inlet 15 to the gap betweenfluid handling structure 12 and substrate W. The gas is extracted viaoutlet 14. The overpressure on the gas inlet 15, vacuum level on theoutlet 14 and geometry of the gap are arranged so that there is ahigh-velocity gas flow 16 inwardly that confines the liquid. The forceof the gas on the liquid between the fluid handling structure 12 and thesubstrate W contains the liquid in a space 11. The inlets/outlets may beannular grooves which surround the space 11. The annular grooves may becontinuous or discontinuous. The flow of gas 16 is effective to containthe liquid in the space 11. Such a system is disclosed in United Statespatent application publication no. US 2004-0207824.

The example of FIG. 5 is a localized area arrangement in which liquid isonly provided to a localized area of the top surface of the substrate Wat any one time. Other arrangements are possible, including fluidhandling systems which make use of a single phase extractor or a twophase extractor as disclosed, for example, in United States patentapplication publication no US 2006-0038968.

Another arrangement which is possible is one which works on a gas dragprinciple. The so-called gas drag principle has been described, forexample, in United States patent application publication nos. US2008-0212046, US 2009-0279060, and US 2009-0279062. In that system theextraction holes are arranged in a shape which desirably has a corner.The corner may be aligned with the stepping or scanning directions. Thisreduces the force on the meniscus between two openings in the surface ofthe fluid handing structure for a given speed in the step or scandirection compared to a fluid handling structure having two outletsaligned perpendicular to the direction of scan.

Also disclosed in US 2008-0212046 is a gas knife positioned radiallyoutside the main liquid retrieval feature. The gas knife traps anyliquid which gets past the main liquid retrieval feature. Such a gasknife may be present in a so called gas drag principle arrangement (asdisclosed in US 2008-0212046), in a single or two phase extractorarrangement (such as disclosed in United States patent applicationpublication no. US 2009-0262318) or any other arrangement.

Many other types of liquid supply system are possible. The presentinvention is neither limited to any particular type of liquid supplysystem, nor to immersion lithography. The invention may be appliedequally in any lithography. In an EUV lithography apparatus, the beampath is substantially evacuated and immersion arrangements describedabove are not used.

A control system 500 shown in FIG. 1 controls the overall operations ofthe lithographic apparatus and in particular performs an optimizationprocess described further below. Control system 500 can be embodied as asuitably-programmed general purpose computer comprising a centralprocessing unit and volatile and non-volatile storage. Optionally, thecontrol system may further comprise one or more input and output devicessuch as a keyboard and screen, one or more network connections and/orone or more interfaces to the various parts of the lithographicapparatus. It will be appreciated that a one-to-one relationship betweencontrolling computer and lithographic apparatus is not necessary. In anembodiment of the invention one computer can control multiplelithographic apparatuses. In an embodiment of the invention, multiplenetworked computers can be used to control one lithographic apparatus.The control system 500 may also be configured to control one or moreassociated process devices and substrate handling devices in a lithocellor cluster of which the lithographic apparatus forms a part. The controlsystem 500 can also be configured to be subordinate to a supervisorycontrol system of a lithocell or cluster and/or an overall controlsystem of a fab.

FIG. 6 schematically depicts an EUV lithographic apparatus 4100including a source collector apparatus SO. The apparatus comprises:

an illumination system (illuminator) EIL configured to condition aradiation beam B (e.g. EUV radiation);

a support structure (e.g. a mask table) MT constructed to support apatterning device (e.g. a mask or a reticle) MA and connected to a firstpositioner PM configured to accurately position the patterning device;

a substrate table (e.g. a wafer table) WT constructed to hold asubstrate (e.g. a resist-coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate; and

a projection system (e.g. a reflective projection system) PS configuredto project a pattern imparted to the radiation beam B by patterningdevice MA onto a target portion C (e.g. comprising one or more dies) ofthe substrate W.

These basic components of the EUV lithographic apparatus are similar infunction to the corresponding components of the lithographic apparatusof FIG. 1. The description below mainly covers areas of difference andduplicative description of aspects of the components that are the sameis omitted.

In an EUV lithographic apparatus, it is desirable to use a vacuum or lowpressure environment since gases can absorb too much radiation. A vacuumenvironment can therefore be provided to the whole beam path with theaid of a vacuum wall and one or more vacuum pumps.

Referring to FIG. 6, the EUV illuminator EIL receives an extreme ultraviolet radiation beam from the source collector apparatus SO. Methods toproduce EUV radiation include, but are not necessarily limited to,converting a material into a plasma state that has at least one element,e.g., xenon, lithium or tin, with one or more emission lines in the EUVrange. In one such method, often termed laser produced plasma (“LPP”)the plasma can be produced by irradiating a fuel, such as a droplet,stream or cluster of material having the desired line-emitting element,with a laser beam. The source collector apparatus SO may be part of anEUV radiation system including a laser, not shown in FIG. 6, to providethe laser beam exciting the fuel. The resulting plasma emits outputradiation, e.g., EUV radiation, which is collected using a radiationcollector, disposed in the source collector apparatus. The laser and thesource collector apparatus may be separate entities, for example when aCO₂ laser is used to provide the laser beam for fuel excitation.

In such cases, the laser is not considered to form part of thelithographic apparatus and the radiation beam is passed from the laserto the source collector apparatus with the aid of a beam delivery systemcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thesource collector apparatus, for example when the source is adischarge-produced plasma EUV generator, often termed as a DPP source.

The EUV illuminator EIL may comprise an adjuster to adjust the angularintensity distribution of the radiation beam EB. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the EUV illuminator EILmay comprise various other components, such as facetted field and pupilmirror devices. The EUV illuminator EIL may be used to condition theradiation beam EB, to have a desired uniformity and intensitydistribution in its cross section.

The radiation beam EB is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., mask table) MT, and ispatterned by the patterning device. After being reflected from thepatterning device (e.g. mask) MA, the radiation beam EB passes throughthe projection system PS, which focuses the beam onto a target portion Cof the substrate W. With the aid of the second positioner PW andposition sensor PS2 (e.g. an interferometric device, linear encoder orcapacitive sensor), the substrate table WT can be moved accurately, e.g.so as to position different target portions C in the path of theradiation beam EB. Similarly, the first positioner PM and anotherposition sensor PS1 can be used to accurately position the patterningdevice (e.g. mask) MA with respect to the path of the radiation beam EB.Patterning device (e.g. mask) MA and substrate W may be aligned usingmask alignment marks M1, M2 and substrate alignment marks P1, P2.

The depicted apparatus could be used the same modes as the apparatus ofFIG. 1.

FIG. 7 shows the EUV apparatus 4100 in more detail, including the sourcecollector apparatus SO, the EUV illumination system EIL, and theprojection system PS. The source collector apparatus SO is constructedand arranged such that a vacuum environment can be maintained in anenclosing structure 4220 of the source collector apparatus SO. An EUVradiation emitting plasma 4210 may be formed by a discharge producedplasma source. EUV radiation may be produced by a gas or vapor, forexample Xe gas, Li vapor or Sn vapor in which the plasma 4210 is createdto emit radiation in the EUV range of the electromagnetic spectrum. Theplasma 4210 is created by, for example, an electrical discharge causingan at least partially ionized plasma. Partial pressures of, for example,10 Pa of Xe, Li, Sn vapor or any other suitable gas or vapor may berequired for efficient generation of the radiation. In an embodiment, aplasma of excited tin (Sn) is provided to produce EUV radiation.

The radiation emitted by the plasma 4210 is passed from a source chamber4211 into a collector chamber 4212 via an optional gas barrier and/orcontaminant trap 4230 (in some cases also referred to as contaminantbarrier or foil trap) which is positioned in or behind an opening insource chamber 4211. The contaminant trap 4230 may include a channelstructure. Contamination trap 4230 may also include a gas barrier or acombination of a gas barrier and a channel structure. The contaminanttrap or contaminant barrier 4230 further indicated herein at leastincludes a channel structure, as known in the art.

The collector chamber 4212 may include a radiation collector CO whichmay be a so-called grazing incidence collector. Radiation collector COhas an upstream radiation collector side 4251 and a downstream radiationcollector side 4252. Radiation that traverses collector CO can bereflected by a grating spectral filter 4240 to be focused in a virtualsource point IF. The virtual source point IF is commonly referred to asthe intermediate focus, and the source collector apparatus is arrangedsuch that the intermediate focus IF is located at or near an opening4221 in the enclosing structure 4220. The virtual source point IF is animage of the radiation emitting plasma 4210.

Subsequently the radiation traverses the illumination system IL, whichmay include a facetted field mirror device 422 and a facetted pupilmirror device 424 arranged to provide a desired angular distribution ofthe radiation beam 421, at the patterning device MA, as well as adesired uniformity of radiation intensity at the patterning device MA.Upon reflection of the beam of radiation 421 at the patterning deviceMA, held by the support structure MT, a patterned beam 426 is formed andthe patterned beam 426 is imaged by the projection system PS viareflective elements 428, 430 onto a substrate W held by the substratestage or substrate table WT.

More elements than shown may generally be present in illumination opticsunit IL and projection system PS. The grating spectral filter 4240 mayoptionally be present, depending upon the type of lithographicapparatus. There may be more mirrors present than those shown in theFigures, for example there may be from 1 to 6 additional reflectiveelements present in the projection system PS than shown in FIG. 7.

Collector optic CO, as illustrated in FIG. 7, is depicted as a nestedcollector with grazing incidence reflectors 4253, 4254 and 4255, just asan example of a collector (or collector mirror). The grazing incidencereflectors 4253, 4254 and 4255 are disposed axially symmetric around anoptical axis O and a collector optic CO of this type is preferably usedin combination with a discharge produced plasma source, often called aDPP source.

Alternatively, the source collector apparatus SO may be part of an LPPradiation system as shown in FIG. 8. A laser LA is arranged to depositlaser energy into a fuel, such as xenon (Xe), tin (Sn) or lithium (Li),creating the highly ionized plasma 4210 with electron temperatures ofseveral 10's of eV. The energetic radiation generated duringde-excitation and recombination of these ions is emitted from theplasma, collected by a near normal incidence collector optic CO andfocused onto the opening 4221 in the enclosing structure 4220.

FIG. 9 depicts a substrate holder according to an embodiment of theinvention. It may be held within a recess in substrate table WT andsupports substrate W. The main body of the substrate holder 100 has theform of a flat plate, for example a disc substantially corresponding inshape and size to the substrate W. The substrate holder can, forexample, be formed from Si, SiC, SiSiC, aluminum nitride (AlN), Zerodur,cordierite, or some other suitable ceramic or glass-ceramic material. Atleast on a top side, in an embodiment on both sides, the substrateholder has projections 106, commonly referred to as burls. In anembodiment, the substrate holder is an integral part of the substratetable and does not have burls on the lower surface. The burls are notshown to scale in FIG. 9. Some or all of the burls are formed by lasersintering as described below.

In a practical embodiment, there can be many hundreds or thousands ofburls, e.g. more than 10,000 or more than 40,000, distributed across asubstrate holder, e.g., of width (e.g., diameter) 200 mm, 300 mm or 450mm. The tips of the burls have a small area, e.g. less than 1 mm². Thusthe total area of all of the burls on one side of the substrate holder100 is less than about 10%, e.g. from 1 to 3%, of the total surface areaof the substrate holder. Because of the burl arrangement, there is ahigh probability that any particle that might lie on the surface of thesubstrate, substrate holder or substrate table will fall between burlsand will not therefore result in a deformation of the substrate orsubstrate holder.

The burl arrangement may form a pattern and/or may have a periodicarrangement. The burl arrangement can be regular or can vary as desiredto provide appropriate distribution of force on the substrate W andsubstrate table WT. The burls can have any shape in plan but arecommonly circular in plan. The burls can have the same shape anddimensions throughout their height but are commonly tapered. Thedistance that the burls project from the rest of the surface of the mainbody 100 a of the substrate holder 100 is in the range of from about 1μm to about 5 mm, desirably from about 5 μm to about 250 μm. Thethickness of the main body 100 a of the substrate holder 100 can be inthe range of about 1 mm to about 50 mm, desirably in the range of about5 mm to 20 mm, typically 10 mm.

Beneficially, the burls may be formed with very consistent dimensions.Desirably the variation between heights of different burls is verysmall. Short burls can be formed (e.g. shorter than 20 μm, shorter than15 μm, shorter than 5 μm or shorter than 3 μm). Shorter burls arebeneficial because they increase the heat transfer between the substrateand the substrate holder. The gap between the top of the substrateholder away from the burls and the supported surface of a substrate onthe substrate holder is smaller than a support with a greater height.Such a small gap facilitates the transfer of heat from a temperatureconditioning element (e.g., heater) to the supported substrate. Theminimum burl height is determined by the variations in the total heightof the thin-film stack and the amount of unflatness of the substrate andsubstrate holder. In an embodiment the burl height is greater than orequal to 1 μm or 2 μm.

The burls can have a width (e.g., diameter) less than or equal to 0.5mm. In an embodiment the burls have a width (e.g., diameter) in therange of from about 200 μm to about 500 μm. The spacing between burls isbetween about 1.5 mm to about 3 mm.

Further, an embodiment of the invention allows use of a wider range ofmaterials for the substrate holder. Materials that are not suitable forprevious methods of forming burls or substrate holders can be used in anembodiment of the invention. In an embodiment, it is possible to usematerial such as cordierite, a low CTE glass-ceramic, which cannoteasily be machined to form burls. Cordierite has good properties for usein a substrate holder. For example, cordierite has a high Young'smodulus of about 140 Gpa and a low thermal conductivity of about 4 W/mK.

A substrate holder manufactured according to an embodiment of theinvention can have a long usable life time due to robust manufacturingmethods. An embodiment of the invention can exhibit desirable wearproperties, for example good wear resistance and therefore lowgeneration of particular contaminants. Beneficially, an embodiment ofthe invention can avoid the need for coating the substrate holder.

A substrate holder according to an embodiment of the invention can havea thin-film component 110 formed on one or both surfaces. A thin-filmcomponent may have a layer thickness in the range of from about 2 nm toabout 100 μm. Such a thin film component may have one or a plurality oflayers. Each layer may be formed by a process including chemical vapordeposition, physical vapor deposition (e.g. sputtering), dip coating,spin coating and/or spray coating. In an embodiment, a component formedon the substrate holder comprises a thin-film stack, i.e. including aplurality of thin-film layers. Such components are described furtherbelow. Although reference in this description is to a thin film stackformed on the top surface of a substrate holder, the thin film stack maybe formed on the undersurface of the substrate holder, or on a substratetable beneath a substrate holder, or on any other surface of thesubstrate table or substrate holder, including a surface of an integralsubstrate holder and substrate table.

An electronic or electric component to be formed on the substrate tablecan include, for example, an electrode, a resistive heater and/or asensor, such as (in a non-limiting list) a strain sensor, a magneticsensor, a pressure sensor, a capacitive sensor or a temperature sensor.A heater and sensor can be used to control and/or monitor locally thetemperature of the substrate holder and/or substrate. Such local controland/or monitoring can reduce undesired, or induce desired, temperaturevariation and stress in the substrate holder or substrate. Desirably,the heater and sensor are formed on, around and/over the same region aseach other. It is desirable to control temperature and/or stress of thesubstrate in order to reduce or eliminate imaging errors such as overlayerrors due to local expansion or contraction of the substrate. Forexample, in an immersion lithography apparatus, evaporation of residualimmersion liquid (e.g., water) on the substrate can cause localizedcooling, may apply a heat load to the surface on which the liquid islocated, and hence shrinkage of the substrate. Conversely, the energydelivered to the substrate by the projection beam during exposure cancause significant heating and therefore expansion of the substrate.

In an embodiment, the component to be formed is an electrode for anelectrostatic clamp. In electrostatic clamping, an electrode provided onthe substrate table and/or substrate holder is raised to a highpotential, e.g. from 10 to 5,000 V. The substrate can be grounded orfloating. Electrostatic forces in the electric field generated by theelectrode attract the substrate to the substrate table and/or holder toprovide a clamping force. This is described further below.

One or more electrical connections can be provided to connect theelectric or electronic component on the substrate holder to a voltagesource (not shown for convenience). If the component is an electrostaticclamp, the electrode on the substrate has an electrical connection tothe voltage source. The component may be on a top surface of thesubstrate support. At least part of the electrical connection may passthrough the body of the substrate support as described in U.S. patentapplication no. 61/555,359, filed on 3 Nov. 2011, which is herebyincorporated by reference in its entirety.

In an embodiment, one or more localized heaters 101 are controlled bycontroller 103 to provide a desired amount of heat to the substrateholder 100 and substrate W to control the temperature of the substrateW. One or more temperature sensors 102 are connected to controller 104which monitors the temperature of the substrate holder 100 and/orsubstrate W. Arrangements using one or more heaters and temperaturesensors to locally control the temperature of a substrate are describedin copending U.S. patent application publication no. US 2012-0013865,which document is incorporated herein by reference in its entirety. Thearrangements described therein can be modified to make use of aresistive heater and temperature sensor as described herein. Furtherdetails of thin-film stacks including components thereof and a method ofmanufacture thereof are given in U.S. patent application Ser. No.13/403,706 filed on 23 Feb. 2012 and co-pending U.S. patent applicationNo. 61/621,648, filed on Apr. 9, 2012, which documents are herebyincorporated by reference in their entireties.

A substrate holder for use in a conventional (e.g., DUV) lithographicapparatus (e.g. an immersion lithographic apparatus) is desirablyprovided with one or more thin-film temperature sensors and/or one ormore thin-film heaters. Other forms of sensor and/or heater can beprovided in, on and/or under the substrate holder.

A substrate holder for use in an EUV lithographic apparatus is desirablyprovided with a thin-film electrostatic clamp and optionally one or morethin-film temperature sensors and/or one or more thin-film heaters.Other forms of sensor and/or heater can be provided in, on and/or underthe substrate holder.

As mentioned, laser sintering may be used to form the burls. This methodis illustrated in FIGS. 10A to E and starts with a flat plate of thedesired shape which forms the main body 400 of the substrate holder. Theflat plate may be pre-formed by another technique. In an embodiment theplate is formed of SiSiC but one or more other materials such as Invar™,Zerodur™, ULE™, fused silica, cordierite, boron nitride, siliconnitride, aluminum nitride (AlN) and/or SiC can be used. Desirably, asurface 400 a of the plate is ground and/or polished to a desired degreeof flatness. In an embodiment, the surface is cleaned, e.g. with ozone,but this step can be omitted. In an embodiment, the surface 400 a istreated to promote adherence of one or more subsequent layers, e.g. byapplication of a primer layer, but this step can be omitted. On theplate, an isolation layer 410 is applied to isolate one or more metallayers to be formed above it from the main body of the substrate holder.In an embodiment, the isolation layer 410 improves flatness. Theisolation layer 410 may be made of BCB applied by spin or spray coatingas described above or of SiO₂ applied by a PECVD process, or othersuitable material. On top of the isolation layer, a metal layer 440 isapplied, e.g. by PVD, to arrive at the situation shown in FIG. 10A.

The metal layer is then patterned, e.g. by lithography and selectiveetching, e.g. a wet etch, to define the desired pattern to form adesired component, e.g. an electrode, a sensor or a heater. This stepalso removes the metal layer in an area where burls are to be formed ina subsequent step. At this stage, the substrate holder is as illustratedin FIG. 10B.

Over the patterned metal layer, an isolation or dielectric layer 450 isapplied and an opening through to the main body or a base layer, i.e.through both isolation layers, are formed in locations where burls aredesired. The substrate holder is now as illustrated in FIG. 10C.Optionally, the exposed areas 400 b of the surface of the main body 400are cleaned, e.g. with ozone, and/or treated, e.g. by application of aprimer layer to promote adhesion of the burls which are to be formedsubsequently.

Burls 406 are now formed in the opening through the thin film stack by alaser sintering process. Other structures on the substrate holder, forexample a vacuum ring, can be formed concurrently with the burls. It isalso possible to form one or more projections between the burls that areshorter than the burls but have a larger area. Such a projectionimproves thermal transfer between the substrate and the substrateholder. Such a projection can be, for example 10 μm or more shorter thanthe burls 406. There are two types of laser sintering methods, both areusable.

In the first method, a thin layer of powder is applied to the area whereburls are to be formed. Then one or more laser beams are used toselectively sinter the powder in the area where the burls are to beformed. When that is complete, another thin layer of powder is appliedand selectively heated and sintered. This is repeated so that the burlis built up layer by layer. In an embodiment, each layer has a thicknessin the range of from 1 to 1.5 μm. Since the sintering pattern can bevaried at each layer, the burl can be built up with any desired shapeand/or profile. In this method, the powder may be applied over a largearea and multiple burls formed simultaneously or concurrently.Alternatively, powder may be applied to a small area and each burlformed independently. Further details of this process can be found in“Laser micro sintering—a quality leap through improvement of powderpacking” by A Streek et al published athttp://laz.htwm.de/43_rapidmicro/55_Ver%C3%B6ffentlichungen/Laser%20micro%20sintering%20%20a%20quality%20leap%20through%20improvement%20of%20powder%20packing.pdf.

In the second method, powder is jetted in an inert gas over the areawhere a burl is to be formed while one or more laser beams irradiate theprecise locations where burls are to be formed. Powder selectivelyadheres to the positions irradiated by the laser beam. By suitablyshifting the point of irradiation, a burl of desired profile can bebuilt up. Further details of this process can be found in“MICRO-CLADDING USING A PULSED FIBRE LASER AND SCANNER” by S. Kloetar etal published athttp://laz.htwm.de/43_rapidmicro/55_Ver%C3%B6ffentlichungen/Microcladding_LPM2010.pdf.

As with other sintering techniques, laser sintering works by partiallymelting particles of the powder so that they adhere together when theycool. Laser sintering has an advantage in that the controlledapplication of the laser beam allows for spatial control of wheresintering takes place. In both methods described above, the powder canbe pre-heated to a temperature close to the relevant melting point sothat less energy need be applied by the laser beam to complete thesintering. A wide variety of materials can be used in sinteringtechniques. The powder can be formed of a single material, e.g. a metalsuch as titanium, a semiconductor such as silicon or a ceramic such asfused silica, cordierite and/or aluminum nitride. In an embodiment, thepowder is made of two or more components. One component has a relativelylow melting point which melts to form a matrix in which the otherparticulate component(s) is(are) embedded. The matrix-forming componentof the powder can be provided as separate particles or as a coating onparticles of another material(s). The matrix forming compound can be anyof the single materials mentioned above. The particulate component canbe one or more components selected from the group comprising cubic boronnitride, silicon nitride, silicon carbide, titanium nitride, titaniumcarbide and/or diamond, e.g. diamond-like carbon (DLC). The sinteringprocess can be carried out in an inert atmosphere or a vacuum to helpprevent chemical change to the material being sintered or in acontrolled atmosphere to promote a chemical change.

Thus, the material from which the burl is to be formed can be selectedfrom a wide range of materials to provide a desired property such asstrength of adherence to the material of the base body of the substrateholder. Desirably, the burl is made of the same material as, or amaterial compatible with, the material of the main body of the substrateholder. For example, it is generally desirable that the burl bond wellto the base material of the main body of the substrate so as to providelongevity and robustness in use. In some applications, it is desirablethat the burls have high thermal conductivity to assist in temperatureconditioning of the substrate. In other applications, a low thermalconductivity can be desirable in order to isolate the substrate. Otherrelevant properties of the burls that can be affected through choice ofmaterial include electrical conductivity, dielectric strength and wearresistance.

The laser sintering technique for forming the burls generally results ina rough upper surface to the burls as depicted in FIG. 10D. If so, it isdesirable to perform a final polishing step so as to provide a smoothupper surface to the burls as illustrated FIG. 10E. In some cases, e.g.if the final polishing is performed with a coarse-grained slurry, itmight be desirable to first protect the thin film stacks with anadditional coating. However this is often not necessary, for examplewhere the thin film stack contains only electrodes for clampingpurposes.

A further advantage of the laser sintering process is that it allows thecomposition of a burl to be varied through its height. It is thereforepossible to manufacture burls having one or more sections or layers ofdifferent composition and/or property as illustrated in FIG. 11. Forexample, a lower part 406 a of a burl can be formed of material thatbonds well to the material of the base body of the substrate holder,while the upper part 406 b of the burl is formed of a material having,e.g., an improved wear property. (Note that the base body may be madeusing a different technique from the burls.) For example, particles ofdiamond, such as diamond-like carbon (DLC), can be included in the upperpart 406 b of the burl to improve wear resistance. Alternatively,diamond particles (e.g. DLC) can be included in the lower part 406 a toimprove thermal conductivity. In an embodiment, a burl is formed withmore than two distinct layers. In an embodiment, a burl is formed with agradual change in composition, content or material property through atleast a part of its height.

It is also possible to vary the composition of the powder to be sinteredin a direction substantially parallel to the surface on which the burlis being formed. In the powder layer method of sintering, this can beachieved through variation of the composition of the powder within eachlayer of powder as it is applied. In the powder jetting method, this canbe achieved through variation of the composition of the jetted powderwith time in synchronization with movement of the point of laserirradiation. Varying the material composition of the burl in a directionsubstantially parallel to the surface on which it is formed, optionallyin addition to variation in the height direction, can allow fine controlover one or more mechanical and other properties of the burl, e.g.stiffness.

An advantage of an embodiment of the invention is that burls can beformed with almost any shape in three dimensions. In an embodiment, aburl has a constant cross-section throughout its height. In anembodiment, a burl tapers away from the main body of the substrateholder. In an embodiment, the cross-section of a burl varies withheight. In an embodiment, a burl has a cross-section, substantiallyparallel to the surface of the main body of the substrate holder, thatis selected from the group consisting of circle, square, rectangle,oval, rhombus and “racetrack” or “stadium” shape. A “racetrack” or“stadium” shape has two straight parallel sides joined by curves, e.g.semicircles.

In an embodiment, a thin-film stack is provided on only one side of thesubstrate holder. In an embodiment, no thin film stacks are provided onthe substrate holder. In an embodiment, thin-film stacks are provided onboth sides of the substrate holder. In an embodiment, burls are providedon both sides of the substrate holder. If burls are provided on bothsides of the substrate holder it is not necessary that the same methodof forming the burls is used on both sides.

An embodiment of the present invention can be employed to form burls onother components in a lithographic apparatus where controlled contactbetween a component of the apparatus and an object to be handled isdesired.

FIG. 12 shows an embodiment of the invention which is a supportstructure for a patterning device, for example a mask table MT tosupport a mask MA. The main body 500 of the mask table is, in contrastwith a substrate table, provided with a through hole 501 for passage forthe projection beam B. The mask MA is supported by burl areas 502 formedby laser sintering on the mask table 500. The use of burls to supportthe mask MA has similar functions of the use of burls to support asubstrate on the substrate table or holder. For example, burls canprevent or reduce the chance of the mask being misaligned or distortedby the presence of a particle between the mask and a surface on which itis supported. Burls can allow a mask to be clamped to a mask table orother support structure using a vacuum and/or electrostatic clampingtechnique. Since in most lithographic apparatuses the projection systemPS reduces the projected image by a factor of 4 or 5, in a scanning modeof operation, the mask is moved with a velocity and acceleration 4 or 5times greater than the substrate table. Therefore, a commensuratelylarger clamping force should be exerted. Desirably, a mask is held onthe support structure in such a manner that no or minimal stress isgenerated in the mask. The use of burls can contribute to this aim.

In FIG. 12, burls are shown as provided in discrete areas either side ofthe aperture 501. In an embodiment, burls can be provided in acontinuous area completely surrounding the aperture or in multiplelocations spaced around the aperture. The exact location of the burlscan be determined in construction of an embodiment. Burls used tosupport a mask, e.g. on a mask table or mask handling device, can have adiameter of from 100 μm to 1 μm, desirably 300 to 500 μm. The burls canhave an elongate shape in plan such as an ellipse or rectangle.

In an embodiment, burls are formed by laser sintering on a clamp for amask or reticle. The clamp can be a layered structure of, for example, aULE-Zeodur-ULE or a thin film stack-Zerodur-thin film stack. Such aclamp can have a thickness of about 10 mm. The burls can have a pitch offrom 2 to 10 mm.

FIG. 13 depicts in plan a support structure for a patterning device,e.g. a mask or a reticle, according to an embodiment of the invention.FIG. 14 depicts the same support structure in cross-section along lineXX of FIG. 13. The support structure comprises a support member 512 thatis supported above a main body 510 by a leaf spring 513. The uppersurface of the support member 512 has a central recess 515 surrounded byan area of burls 106. The central recess 515 is connected via conduit516 to a low pressure (e.g., vacuum) source 517.

When a patterning device such as mask MA is placed on support member 512and pressure in the central recess 515 is reduced, the patterning deviceis clamped securely in place. By virtue of the leaf spring 513, theposition of the support member 512 in a direction substantiallyperpendicular to the surface of the patterning device MA is welldefined. However, the support member 512 is allowed to move in at leastone direction substantially parallel to the surface of the patterningdevice MA. This means that it help ensure that no or minimal stressesarise in the patterning device MA. Such stresses are undesirable as theymight distort the pattern defined by patterning device MA. The provisionof burls 106 formed by laser sintering on the support member 512 helpsensure that the patterning device MA is accurately positioned.

FIG. 15 illustrates a substrate handling arm or gripper 600 according toan embodiment of the invention. Substrate handling arm 600 is driven byone or more actuators (not shown). Substrate handling arm 600 is used totransfer a substrate between, e.g., a loading dock and a pre-alignmentstage, between the pre-alignment stage and the substrate table and/orbetween the substrate table and an unloading dock. Similar handlingdevices may be used in the track part of a lithocell or may be used tomove a patterning device (e.g., a mask). The substrate handling arm 600comprises a pair of fingers or prongs 601 spaced apart in asubstantially horizontal plane. An upper surface, or part thereof, ofeach of the prongs 601 is provided with an area 602 having burls formedby laser sintering. The use of burls on the substrate handling arm canhave one or more of the same advantages as described above, that is,e.g., enabling the use of a vacuum and/or electrostatic clampingtechnique and/or preventing particulates distorting the substrate.

FIG. 16 shows an embodiment of the invention in which a substrate holderis integrated with the substrate table WT. The holder WT has a main body700 in which is formed a recess 701 within which the substrate W can beaccommodated. The lower surface of the recess 701 is provided with burls106 formed by laser sintering described above. The depth of the recess701 and height of burls 106 are determined to help ensure that the uppersurface of the substrate W is substantially coplanar with the uppersurface of the main body 700. In this way, one or more sensors, such asa transmission image sensor TIS, provided in the substrate table WT canmake measurements at a substantially same vertical position as theexposures onto the substrate W will be performed.

An advantage of embodiment of the present invention is that burls can beformed reliably and accurately on a wider variety of surfaces thanpreviously known methods of forming burls. Thus, burls can be providedon a component that would not have been suitable for provision of burlsby prior techniques such as a material removal technique, for exampleelectric discharge machining (EDM).

A further advantage is that methods described herein can be used torepair burls, individually or collectively. Two methods of repairingburls according to embodiments of the invention will be described below.These methods can be used to repair burls formed by any method, not justburls formed by laser sintering.

In a repair method according to an embodiment of the invention, steps ofwhich are illustrated in FIGS. 17A to 17C, individual burls arerepaired. As shown in FIG. 17A, an object holder 800 has a plurality ofburls 801 and a damaged burl 802. The damaged burl 802 has a rough uppersurface 802 a caused by, for example, wear or physical damage. Thedamaged burl is further ground down to provide a smooth surface 802 b asshown in FIG. 17B. A temporary protective coating can be provided tocover intact burls 801 during this process. If desired, the groundsurface 802 b can be cleaned, e.g. with ozone, and/or pre-treated, e.g.by application of a primer. A laser sintering technique, as describedabove, is next used to rebuild the burl to its original shape and/orheight (or to a different desired shape and/or height). This may involvebuilding the repaired burl 803 to a height higher than the ultimatelydesigned height and polishing back to a level matching the height ofintact burls 801, as shown in FIG. 17C.

In a further repair method, steps of which are depicted in FIG. 18A to18C, a plurality of burls 901 on an object holder 900 are repairedsimultaneously. This approach is particularly applicable for theperiodic rehabilitation of an object holder to account for wear ratherthan to address isolated incidences of damage. This method can also beused to adjust the height of burls to suit changed operating parameters,e.g. a change in thickness of a substrate to be exposed.

In the method of this embodiment, the upper surfaces of burls 901, shownin FIG. 18A, can optionally be cleaned, e.g. using ozone, or prepared,e.g. by application of a primer. Then an additional layer 902 is builtup on the burls 901 by a laser sintering process as described above. Inan embodiment, the additional layer 902 has a thickness in the range offrom 1 to 5 μm, desirably 2 to 3 μm. In most cases this will provide arough upper surface 902 a, shown in FIG. 18B, which is at a higherheight than the ultimate desired height for the burls. The burls canthen be polished back to provide a desired flatness and desiredroughness of top surface 902 b at the desired height, as shown in FIG.18C.

As well as a direct benefit of providing a method of repairing burls,namely that damaged components do not always need to be replaced, theexistence of a repair technique enables burls to be formed directly on acomponent where it would be economically undesirable to replace thecomponent in the invent of damage to one or more burls.

An embodiment of the present invention advantageously enables the use ofburls on a component of a lithographic apparatus such as a substratetable, a support structure for a patterning device, a mask table, awafer handler, a mask handler, a gripper, a pre-alignment stage, aprocess device in a track, a substrate handling robot, a conditioningplate, a substrate conditioning unit and/or a sensor mount. Burlsmanufactured according to an embodiment of the invention can be usedwherever an object—such as a substrate, patterning device, opticalelement or sensor—is to be held or mounted at a precise location. Asensor which can be mounted on burls formed according to an embodimentof the invention may include a transmission image sensor and/or aninterferometric aberration sensor.

An advantage of embodiment of the present invention is that it can formburls more accurately than a subtractive technique such as electricdischarge machining. With an embodiment of the invention, burls may beformed reliably and accurately. Any burls that are missing or damaged inthe manufacturing process can be easily added or repaired. In anembodiment the body may be made by a different technique from the burlsformed thereon.

As will be appreciated, any of the above described features can be usedwith any other feature and it is not only those combinations explicitlydescribed which are covered in this application.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications in manufacturing components with microscale, or evennanoscale features, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 248, 193, 157 or 126 nm).

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, including refractiveand reflective optical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention, at least in the form of amethod of operation of an apparatus as herein described, may bepracticed otherwise than as described. For example, the embodiments ofthe invention, at least in the form of a method of operation of anapparatus, may take the form of one or more computer programs containingone or more sequences of machine-readable instructions describing amethod of operating an apparatus as discussed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein. Further, the machine readableinstruction may be embodied in two or more computer programs. The two ormore computer programs may be stored on one or more different memoriesand/or data storage media.

Any controllers described herein may each or in combination be operablewhen the one or more computer programs are read by one or more computerprocessors located within at least one component of the lithographicapparatus. The controllers may each or in combination have any suitableconfiguration for receiving, processing and sending signals. One or moremultiple processors are configured to communicate with at least one ofthe controllers. For example, each controller may include one or moreprocessors for executing the computer programs that includemachine-readable instructions for the methods of operating an apparatusas described above. The controllers may include data storage media forstoring such computer programs, and/or hardware to receive such media.So the controller(s) may operate according to the machine readableinstructions of one or more computer programs.

An embodiment of the invention may be applied to substrates with a width(e.g., diameter) of 300 mm or 450 mm or any other size.

One or more embodiments of the invention may be applied to any immersionlithography apparatus, in particular, but not exclusively, those typesmentioned above, whether the immersion liquid is provided in the form ofa bath, only on a localized surface area of the substrate, or isunconfined on the substrate and/or substrate table. In an unconfinedarrangement, the immersion liquid may flow over the surface of thesubstrate and/or substrate table so that substantially the entireuncovered surface of the substrate table and/or substrate is wetted. Insuch an unconfined immersion system, the liquid supply system may notconfine the immersion liquid or it may provide a proportion of immersionliquid confinement, but not substantially complete confinement of theimmersion liquid.

A liquid supply system as contemplated herein should be broadlyconstrued. In certain embodiments, it may be a mechanism or combinationof structures that provides a liquid to a space between the projectionsystem and the substrate and/or substrate table. It may comprise acombination of one or more structures, one or more liquid inlets, one ormore gas inlets, one or more gas outlets, and/or one or more liquidoutlets that provide liquid to the space. In an embodiment, a surface ofthe space may be a portion of the substrate and/or substrate table, or asurface of the space may completely cover a surface of the substrateand/or substrate table, or the space may envelop the substrate and/orsubstrate table. The liquid supply system may optionally further includeone or more elements to control the position, quantity, quality, shape,flow rate or any other features of the liquid.

In a first aspect of the invention there is provided a method ofmanufacturing an object holder for use in a lithographic apparatus, themethod comprising: providing a main body having a surface; and forming aplurality of burls on the surface, the burls projecting from the surfaceand having end surfaces to support an object, wherein forming at leastpart of at least one of the burls comprises laser-sintering.

The laser-sintering may comprise: applying a layer of powder to thesurface; and selectively irradiating the layer of powder with aradiation beam so as to cause at least partial melting of the powder atirradiated locations. In an embodiment, the laser-sintering comprises:irradiating a location on the surface; and jetting powder at theirradiated location.

The main body may be formed of a different material than the at leastone burl. The at least one burl may comprise at least one materialselected from the group consisting of: Ti, Si, fused silica, Cordierite,diamond-like carbon, SiC, SiO₂, AlN, TiN and CrN. The at least one burlmay be formed of a matrix material and particles embedded in the matrixmaterial. The matrix material may comprise at least one materialselected from the group consisting of: Ti, Si, fused silica, Cordierite,diamond-like carbon, SiC, SiO₂, AlN, TiN and CrN. The particles maycomprise at least one material selected from the group consisting of:cubic boron nitride, silicon nitride, silicon carbide, titanium nitride,titanium carbide and diamond.

At least one burl may comprise a first layer of a first material and asecond layer of a second material that is different from the firstmaterial. The first and second materials may be different in a propertyor a component.

In an embodiment, the object is a substrate or a patterning device. Athin film stack may be provided on the surface, the plurality of burlsprojecting further from the surface than the thin film stack.

In a second aspect of the invention there is provided a method ofrepairing an object holder having burls for use in a lithographicapparatus, the method comprising: preparing a burl to be repaired;applying a layer of material to the burl to be repaired bylaser-sintering; and polishing the layer of material.

In a third aspect of the invention there is provided an object holderfor use in a lithographic apparatus, the object holder comprising: amain body having a surface; and a plurality of burls provided on thesurface and having end surfaces to support an object, wherein at leastpart of at least one of the burls has been formed by laser-sintering.

In an embodiment, the main body is formed of a different material thanthe part of the burl. The at least one burl may comprise at least onematerial selected from the group consisting of: Ti, Si, fused silica,cordierite, diamond-like carbon, SiC, SiO₂, AlN, TiN and CrN. The atleast one burl may be formed of a matrix material and particles embeddedin the matrix material. The matrix material may comprise at least onematerial selected from the group consisting of: Ti, Si, fused silica,cordierite, diamond-like carbon, SiC, SiO₂, ALN, TiN and CrN. Theparticles may comprise at least one material selected from the groupconsisting of cubic boron nitride, silicon nitride, silicon carbide,titanium nitride, titanium carbide and diamond.

At least one burl may comprise a first layer of a first material and asecond layer of a second material that is different from the firstmaterial. The first and second materials may be different in a propertyor a component. The at least one burl may have a substantially constantcross-section substantially parallel to the surface. The at least oneburl may taper away from the surface. A cross-section, substantiallyparallel to the surface, of the at least one burl may be selected fromthe group consisting of: circle, square, rectangle, oval, rhombus and“racetrack” or “stadium” shape. The main body may comprise at least onematerial selected from the group consisting of: Zerodur, cordierite,SiC, SiSiC, AlN, Invar, ceramic and glass-ceramic.

The object holder is constructed and arranged to support, as the object,at least one selected from: a substrate, a patterning device, a sensor,and optical element. In an embodiment the object is a substrate and theobject holder is a substrate holder. The substrate holder may have adiameter substantially equal to 200 mm, 300 mm or 450 mm. The object maybe a patterning device and the object holder is a support structure forthe patterning device. The object holder may be a reticle clamp. Theobject holder may be a substrate gripper. A thin film stack may beprovided on the surface, the plurality of burls projecting further fromthe surface than the thin film stack.

In a fourth aspect of the invention there is provided a lithographicapparatus, comprising: a support structure configured to support apatterning device; a projection system arranged to project a beampatterned by the patterning device onto a substrate; and a substrateholder arranged to hold the substrate, the substrate holder beingaccording to features of the third aspect of the invention.

The lithographic apparatus may comprise a substrate table and whereinthe substrate holder is integrated into the substrate table.

In a fifth aspect of the invention there is provided a table for use ina lithographic apparatus, the table comprising: a main body having asurface; and a plurality of burls on the surface and having end surfacesto support an object, for example a substrate, wherein the burls havebeen formed by laser-sintering.

In a sixth aspect of the invention there is provided a lithographicapparatus, comprising: a support structure configured to support apatterning device; a projection system arranged to project a beampatterned by the patterning device onto a substrate; and a tableaccording to the fifth aspect of the invention.

In a seventh aspect of the invention, there is provided a devicemanufacturing method using a lithographic apparatus, the methodcomprising: projecting a beam patterned by a patterning device onto asubstrate while holding the substrate on a substrate holder, wherein thesubstrate holder comprises: a main body having a surface; and aplurality of burls on the surface and having end surfaces to support thesubstrate, wherein the burls have been formed by laser-sintering.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A method of manufacturing an object holder for use in a lithographicapparatus, the method comprising: providing a main body having a flatsurface; and forming a plurality of burls on the flat surface, the burlsprojecting from the flat surface and having end surfaces to support anobject, wherein forming at least part of at least one of the burlscomprises laser-sintering.
 2. The method of claim 1, wherein thelaser-sintering comprises: applying a layer of powder to the flatsurface; and selectively irradiating the layer of powder with aradiation beam so as to cause at least partial melting of the powder atirradiated locations.
 3. The method of claim 1, wherein thelaser-sintering comprises: irradiating a location on the surface; andjetting powder at the irradiated location.
 4. The method of claim 1,wherein the main body is formed of a different material than the atleast one burl.
 5. The method of claim 1, wherein the at least one burlcomprises at least one material selected from the group consisting of:Ti, Si, fused silica, Cordierite, diamond-like carbon, SiC, SiO₂, AlN,TiN and CrN.
 6. The method of claim 1, to wherein the at least one burlis formed of a matrix material and particles embedded in the matrixmaterial.
 7. The method of claim 1, wherein at least one burl comprisesa first layer of a first material and a second layer of a secondmaterial that is different from the first material.
 8. The method ofclaim 7, wherein the first and second materials are different in aproperty or a component.
 9. The method of claim 1, wherein the object isa substrate or a patterning device.
 10. The method of claim 1, where athin film stack is provided on the surface, the plurality of burlsprojecting further from the surface than the thin film stack.
 11. Anobject holder for use in a lithographic apparatus, the object holdercomprising: a main body having a flat surface; and a plurality of burlsprovided on the surface and having end surfaces to support an object,wherein at least part of at least one of the burls has been formed bylaser-sintering.
 12. The object holder of claim 11, wherein the objectholder is constructed and arranged to support, as the object, at leastone selected from: a substrate, a patterning device, a sensor, and/oroptical element.
 13. The object holder of claim 11, where a thin filmstack is provided on the surface, the plurality of burls projectingfurther from the surface than the thin film stack.
 14. The object holderof claim 11, wherein the object holder is at least one of: a substrateholder for a substrate; a support structure for a patterning device; areticle clamp for a reticle; or a substrate gripper for a substrate. 15.A lithographic apparatus, comprising: a support structure configured tosupport a patterning device; a projection system arranged to project abeam patterned by the patterning device onto a substrate; and an objectholder according to claim 11, wherein the object is the substrate. 16.The method of claim 1, wherein the main body is formed of the samematerial as the at least one burl.
 17. The object holder of claim 11,wherein the main body is formed of a different material than the atleast one burl.
 18. The object holder of claim 11, wherein the at leastone burl comprises at least one material selected from the groupconsisting of: Ti, Si, fused silica, Cordierite, diamond-like carbon,SiC, SiO₂, AlN, TiN and CrN.
 19. The object holder of claim 11, whereinthe at least one burl is formed of a matrix material and particlesembedded in the matrix material.
 20. The object holder of claim 11,wherein at least one burl comprises a first layer of a first materialand a second layer of a second material that is different from the firstmaterial.